Loud-speaker



y Q E. w. KELLOGG 1,758,797

LOUD SPEAKER Filed Aug. 2, 1926 Inventor: Edward W. Kel I022,

H is Attorney.

Fatented May 13, 3932 ate, 2

" :1 tea EDWARD W. KELLOGG, OF SCHENECTADY, NEW YORK, ASSIGNOR TOGENERAL ELEC- ERIC COMPANY, A CORPORATION OF NEW YORK LOUD-SPEAKERApplication filed August 2, 1926. eel-m1 No. 126,465.

My present invention relates to apparatus for reproducing sound by meansof electric currents, and more particularly to devices nowcommonly'known as loud speakers.

The object of the present invention is to provide a device of the typedescribed, whereby the reproduction of sound may be made with accuracyand fidelity.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, however, will best be understood from reference to the followingspecification taken in connection with the accompanying drawing in whichFig. 1 is a plan view of'a loud speaker which embodies the features ofmy invention; Fig. 2 is a front elevation showing the diaphragm andmounting therefor; Fig. 3 is an exploded perspective view of the centralcore of the magnet and parts associated therewith; Fig. 4 is a rearperspective View of the diaphragm and the spring supporting meanstherefor; Fig. 5 is a View partly in section and partly in elevation ofthe magnetic system for actuating the diaphragm; and Figs. 6 and 7aredetail views illustrative of the means for supporting the outer endof the diaphragm and the armature respectively.

In my prior application, Serial No. 702,455, filed March 27, 1924, Ihave disclosed a soundreproduction device having a low natural period ofvibration in which a coil, movable in a magnetic field, is employed toeffect the vibration of the diaphragm. While attempts have been made tosecure the results achieved by the apparatus described in the priorapplication by the use of devices having permanent magnets and an ironarmature adapted to vibrate between the poles of the magnets, suchattempts have encountered difiiculties owing to the instability of theiron armature in the mid position. For example, if the iron armature insuch a system is displaced slightly, the magnetic pull tends to increasethe displacement, and this magnetic force, instead of increasing inlinear relation with the deflection, rises very rapidly as the point ofactual contact with the pole pieces is approached. Were the relationbetween mag netic force and displacement of the armature a linearrelation, it would be possible to balance the magnetic force against thestiffness of a spring and in this manner obtain stability, together withsmall net stiffness. But since the relation between pull anddisplacement is not linear, the spring must have large excess stiffnessin order to hold the armature in equilibrium.

In my copending application, Serial No. 152,475, filed Dec. 3, 1926, Ihave described different ways of approaching a balance between a springaction and a magnetic pull. lVhile it is possible, by the use of suchmeans, to obtain a good balance and to secure a sound reproducing devicewhich has the low natural frequency of vibration essential for accuratesound reproduction, it is preferable to employ a magnetic system inwhich this natural instability is not present. Such a system is found inthe arrangement presently to be described, in which an iron armaturemoves parallel to the pole faces and in such manner that the air gapsbetween the stationary pole piece and movable armature are constant inlength but Variable in area.

Referring to the drawing, I have indicated in Figs. 1 to 7 aconstruction whereby my invention may be carried into effect. Theessential features in the device shown are a diaphragm 1, adapted tovibrate as a whole, and a movable annular iron armature 2, mounted onthe diaphragm 1, and suspended in a pair of air gaps formed by twoconcentric magnetic members comprising an annular pole piece 3 and. acore 4, the latter having at oneend thereof a pair of coaxial polepieces 5 and 6. Armature 2 is secured to a ring 7 which in turn isattached to diaphragm 1. A rigid annular member 8 of hard rubber, orother suitable material, is secured to the pole piece 3 in a manner tobe presently described. A supporting ring 9 of flexible material, suchas sheet rubber, is secured to the outer circumference of the diaphragmand is clamped to the ring 8 by a metallic ring 10 and a plurality ofscrews 11.

againstthe enlarged portion thereof which comprises pole pieces 5 and 6,and is secured in a ClIClllill recess in pole piece 3 by means of screws13. A yoke 14 bears against the opposite end of core 4 and is providedwith a screw threaded opening therein. A cap 15 bears against yoke 14.By means of a screw 16 extending through cap 15 and yoke 14 and engagingcore 4, the latter may be placed under tension thereby drawing theenlarged portion of core 4 against the copper ring 12 and at the sametime clamping magnets 17 between yoke 14 and pole piece 3.

An annular adjustable member 18 is secured to pole piece 3 by means of aseries of screws 19 fitting in enlarged openings 20 in the pole piece 3,and is provided with a plurality of flat metal braces 21 which aresecured to the ring 8 and serve to hold it rigidly in position. Polepiece 3 is provided with a plurality of radially extending grooves 22spaced 90 apart and adapted to accommodate flat springs 23 which aresecured at one end to flanges 24 on the annular member 18,

and at the other end to the armature 2. The recess or slot 25 betweenpole pieces 5 and 6 is provided with a winding 26 adapted to be suppliedwith current for actuating the diaa phragm 1.

The movable armature 2 overlaps each of the pole pieces 5 and 6,providing in this manner a pair of long narrow areas of overlap. orworking edges which are desirable in order to obtain a proper degree ofsensitivity. To obtain the best results it is advisable that the totallength of this working edge should exceed 10 cm. I have found that ifthe useful flux alone is instrumental in causing inducthe present caseis considerably less than the mass of the diaphragm 1 to which it isattached. As a further requisite for sensitivity the overlap or air gapwidth should be small. It should, however, exceed by a substantialmargin the. maximum vibration displacement of armature 2 and. shouldalso slightly exceed the air I ap length, that is, the

clearance between a po e piece 5 or 6 and armature 2. Any increase inoverlap beyond the limits indicated adds nothing to the force per ampereturn of winding 26 but does increase the inductance of the latterwinding.

The clearance between armature 2 and pole pieces 5 and 6 also entersinto the question of sensitivity] If the clearance is increased but thefield magnetizing force increased at the same time so as to give thesame mean flux density, the force per ampere turn remains the same whilethe inductance of the winding is decreased. It is, therefore, notdesirable to work with extremely small clearance but to use as large aclearance as is compatible with full magnetic strength and a moderateamount of magnetic material. A clearance of .015 inches and an overlapof .020 inches are indicative of dimensions which may be employed toobtain satisfactory results.

The air gaps on the two sides of the armature are in series making atotal gap of .030 inches. With cobalt steel magnets about five incheslong a good magnetic density (about 10,000 gausses) may be obtainedacross the gaps and owing to the small overlap the total flux is notexcessive and a large cross section of the permanent magnets is,therefore, not required.

The armature 2 is strongly held and centered between the pole pieces 3and 4 by springs 23 so that the armature cannot pull over to one side.This arrangement is readily compatible with great flexibility in thedirection of useful motion. 111 the present instance the springs 23 arein the plane of the ring armature 2. A slight bending of the springspermits motion in a useful direction but the spring material wouldactually have to be stretched to permit any lateral motion. A slight bowis made in each of the springs 23 as indicated in Figs. 4 and 5. Thisconsiderably increases the permissible amplitude of vibration withoutimpairing the ability of the springs to hold the iron ring 2 in acentral position.

The flexible supporting member 9, which is attached to the outer end ofthe diaphragm,

and springs 23 are disposed in planes substantially perpendicular to thedirection of motion of the diaphragm 1 and cooperate to prevent anylateral movement of the diaphragm while at the same time permittingmotion in a longitudinal direction.

With a device of the type described, the quality of reproductionobtained approaches that obtained with the moving coil drive describedin my prior application, Serial No. 702,455, filed March 27, 1924.Although the present device is slightly less responsive to the highfrequencies, I obtain a flexibllit-y sufficient to bring the fundamentalnatural frequency or resonance to a frequency which is less than 150cycles per second when using a six inchdiameter cone of .007 paper, anthis resonance frequency can readily be made as low as 100 to 125cycles.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. In an apparatus for reproducing sound, a diaphragm adapted to vibrateas a whole, an amiular armature of magnetic material mounted thereon, amagnetic member cooperating with the armature, a winding mounted in agroove on said member adapted to actuate said armature and diaphragm,the air gaps between said member and armature being constant in lengthbut variable in area.

2. In an apparatus for reproducing sound,

a diaphragm adapted to vibrate-as a whole, an armature of magneticmaterial mounted thereon, a plurality of pole pieces cooperating withsaid armature, a .winding associated with said pole pieces and adaptedto actuate the diaphragm, said armature overlapping each of said polepieces by an amount greater than the maximum vibratory displacement ofsaid diaphragm. v

3. In an apparatus for reproducing sound,

a diaphragm adapted to vibrate as a whole,

an armature of magnetic material mounted thereon, a plurality of polepieces cooperating with said armature, a winding associated with saidpole pieces and adapted to actuate the diaphragm, said armature.overlapping each of said pole pieces by an amount greater than thedistance between a pole piece and said armature. I

fl. In an apparatus for reproducing sound, a diaphragm, an annulararmature of magnetic material mounted thereon, a core of magneticmaterial cooperating with said diaphragm, said core having an annularrecess at one end thereof, a winding in said recess, said armatureoverlapping each side of said recess by an amount greater than themaximum vibratory displacement of said diaphragm.

-5. In an apparatus for reproducing sound, a diaphragm, an annulararmature of magnetic material mounted thereon, a core of magneticmaterial cooperating with said diaphragm, said core having an annularrecess at one end thereof,a winding in said recess, said armatureoverlapping each side of said recess by an amount which is greater thanthe distance between said core and armature.

6. In an apparatus for reproducing sound, a diaphragm and an armature ofmagnetic material mounted thereon, a magnetic member cooperating withsaid armature, a wind. ing on said member adapted to actuate thediaphragm, said armature overlapping said magnetic member by an amountnot greater than twice the clearance between said member and armature.

7. In an apparatus for reproducing sound, a magnet, a stationary annularwinding mounted adjacent one end thereof, a diaphragm, an annulararmature'of magnetic material mounted 011' said diaphragm and adapted tobe actuted by said winding, said armature being movable in a coaxialdirection with respect to said winding.

8. In an apparatus for reproducing sound, a diaphragm, and means foractuating said diaphragm, said means comprising magnetic members havingoppositely disposed pole faces cooperating toqprovidean air gap, and anarmature of magnetic material attached to said diaphragm and movable insaid air gap in a direction substantially parallel to said cooperatingpole faces, the natural period of vibration of this diaphragm being lessthan 150 cycles per second.

9. In an apparatus for reproducing sound, a diaphragm, an armature ofmagnetic material mounted on said diaphragm, means for actuating saidarmature, said means including a magnet provided with concentric polefaces, said armature being movable between and substantially parallel tosaid pole faces.

10. In an apparatus for reproducing sound, a cone shaped diaphragm,means for supporting the outer end of said diaphragm, an armature ofmagnetic material mounted on the inner end of said diaphragm, means foractuating said diaphragm, said means comprising a magnet pro ided with apole piece having an annular pole face, and a magnetic core having apair of coaxial pole pieces at one end thereof, said coaxial pole piecesbeing concentric with the annular pole face, a winding mounted betweensaid coaxial pole pieces and means whereby current may be applied tosaid winding to actuate said armature.

11. In an apparatus for reproducing sound, a diaphragm, means foractuating said diaphragm, said means comprising a magnet, said magnetincludinga core having an annular recess providing a pair of pole piecesat one end thereof, a winding in said recess, an armature of magneticmaterial mounted on said diaphragm and movable in a direction parallelto the faces of'said pole pieces, and means whereby current may besupplied to the winding to thereby actuate the armature. 1

12. In an apparatusfor reproducing sound, a diaphragm, an annulararmature of magnetic material mounted thereon, means for vibrating saiddiaphragm, said means comprising an annular pole member and a concentriccore member cooperating therewith to provide an annular air gap, and awinding on one of said members.

13. In an apparatus for reproducing sound, a diaphragm, an armaturemounted I 1 thereon, means for actuating sald diaphragm said meansincluding a pair of pole pieces, and means comprising a bowed spring forpreventinglateral movement of the armature while permitting movementthereof in an axial direction.

14. In an apparatus for reproducing sound, a pair of concentric polepieces, a diaphragm, an annular magnetic armature mounted on saiddiaphragm and movable between said pole pieces, and means mounted in theplane of said armature for preventing lateral movement of the armaturewhile permitting movement thereof in an axial direction.

15. In an apparatus for reproducing sound, a pair of pole pieces, adiaphragm, an armature of magnetic material mounted thereon andmovableparallel to the faces of said pole pieces, said armature anddia-.

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phragm being suspended in such a manner that their natural period ofvibration is below'150 cycles per second.

16. In an apparatus for reproducing sound, a diaphragm, an armature ofmagnetic material mounted thereon, a magnetic member cooperating withthe armature, a winding on said member adapted to actuate said armatureand diaphragm, the air-- gaps between said member and said armaturebeing constant-in length but variable in area, and means for supportingthe diaphragm and armature in such a way-that the restoring forceapplied thereto has such relation to the mass of the'diaphragm andarmature that the natural period of vibration-of the diaphragm as aWhole will be less than cycles per second.

17. In an apparatus for reproducing sound, a diaphragm, an armature ofmagnetic material attached thereto, a plurality of pole pieces, saidarmature being movable in a direction parallel to the faces of said polepieces and overlapping each of said pole pieces to provide one or moreworking edges having a total length of at least 10 centimeters.

18. In an apparatus for reproducing sound, a diaphragm, a magneticarmature attached thereto, means including a plurality of pole piecesfor actuating said armature in a direction parallel to the faces of saidpole pieces, and means for preventing lateral movement of the armature,said latter means comprising flexible members attached to the diaphragmand disposed in a plurality of planes which are substantiallyperpendicular to'the direction of motion of the dia phragm.

19. In an apparatus for sound reproduction, a vibratory diaphragm andmeans for producing vibrations thereof comprising a magnetic structurehaving a pair of air gaps, an armature structure of magnetic materialhaving portions extending into both of said air gaps by an amountgreater than the maximum vibratory displacement of said dia phragm, saidarmature structure being so mounted that it is free to move only in adirection substantially parallel to the pole faces which form said airgaps and an actuating coil associated with said magnetic structure insuch a way that currents supplied thereto cause a decrease in the totalflux in one air gap and atthe same time cause an increase in the totalflux in the other air gap. In Witness whereof, I have hereto set my handthis thirtieth day of July, 1926.

EDWARD W. KELLOGG.

